This invention relates to clock and data recovery from a received digital signal in a communications network. The invention further relates to arrangements and methods for determining loss of lock between a receiver and an input signal in an optical communications network.
Digital transmission is widely used for the transport of voice and data signals. In a typical system, the signals from various sources are digitally encoded and are then subjected to one or more levels of multiplexing for transmission over the network. In general, such signals will be transmitted using the SDH (synchronous digital hierarchy) or SONET protocol in which the network is synchronised to a master clock and in which the transmitted signal carries timing information whereby system nodes can synchronise to the master clock. Typically, the higher levels of multiplexing are transmitted in the form of optical signals over fibre paths that provide links between nodes in the network. At the network nodes, received optical signals are converted to corresponding electrical signals for processing, e.g. to effect switching and/or to add/drop channels. In order to perform such operations at a node, it is of course essential to determine the timing or bit rate of the received optical signal so as to affect clock and date recovery and synchronise the node with the signal. This is generally referred to as clock and data extraction. For high bit rate systems, such a function is generally performed by a phase locked loop (PLL) arrangement which synchronises with the received signal clock rate. This then provides a timing reference for the node so as to permit the necessary de-multiplexing of the received signal for the purpose of switching and/or the dropping of channels at the node. A particular problem with currently used phased lock loop circuits is their limited ability to accommodate noisy data. Under noise conditions, such circuits can lose lock with the signal. This causes the generation of a loss of lock alarm and the discarding of the received data signal.
The purpose of loss of lock detection is to discard the data and run the node at a locally generated temporary clock in the event that the received data is in some way faulty and no longer suitable as a timing reference source. In currently used phase locked loop systems, the presence of noise can cause a false alarm of loss of lock resulting in data and timing reference loss even though there may be no fault in the data itself.
An object of the invention is to minimise or to overcome the above disadvantages.
A further object of the invention is to provide an improved arrangement and method for determining loss of lock between an input signal and a receiver in an optical transmission system.
According to a first aspect of the invention, there is provided an optical receiver arrangement comprising: a phase locked loop incorporating an oscillator whereby the loop can be matched in frequency to a received signal frequency, a frequency detector arranged to produce an output indicative of whether the instant loop frequency is above or below said received signal frequency, and control means responsive to said frequency detector output for generating a loss of lock indication when a time averaged measure of difference between the loop frequency and the received signal frequency exceeds a predetermined value.
According to another aspect of the invention, there is provided a network node for use in an optical transmission system in which traffic is transported in multiplexed digital form, the node incorporating optical receiver arrangement comprising: a phase locked loop incorporating an oscillator whereby the loop can be matched in frequency to a received signal frequency, a frequency detector arranged to produce an output indicative of whether the instant loop frequency is above or below said received signal frequency, control means responsive to said frequency detector output for generating a loss of lock indication when a time averaged measure of difference between the loop frequency and the received signal frequency exceeds a predetermined value, and for enabling a local timing means for providing a timing reference to the node.
According to a further aspect of the invention, there is provided a method of detecting loss of lock between a received signal frequency and a local oscillator frequency in an optical receiver arrangement having a phase locked loop incorporating an oscillator whereby the loop can be matched in frequency to a received signal frequency, the method comprising: determining whether the instant loop frequency is above or below said received signal frequency, determining a time averaged measure of a difference between the loop frequency and the received signal frequency, and generating a loss of lock indication when said time averaged difference measure exceeds a predetermined value.
According to a further aspect of the invention, there is provided a circuit arrangement for extracting clock information from a received digital signal and for detecting loss of frequency lock with that signal, the circuit comprising a phase locked loop incorporating an oscillator whereby the loop can be matched in frequency to a received signal frequency, a frequency detector arranged to produce an output indicative of whether the instant loop frequency is above or below said received signal frequency, and control means responsive to said frequency detector output for generating a loss of lock indication when a time averaged measure of difference between the loop frequency and the received signal frequency exceeds a predetermined value.
According to another aspect of the invention, there is provided an arrangement for determining loss of frequency lock between a receiver local oscillator and a received digital optical signal in an optical communications system, the arrangement comprising: a frequency detector arranged to produce an output indicative of whether the instant oscillator frequency is above or below said received signal frequency, and control means responsive to said frequency detector output for generating a loss of lock indication when a time averaged measure of difference between the oscillator frequency and the received signal frequency exceeds a predetermined value.
By providing a time average of frequency slips between the phase locked loop and the received signal, the problem of spurious triggering of a loss of lock alarm by noise or minor glitches in the received signal is substantially overcome.
Advantageously, the frequency detector provides first and second pulse signals indicative of the number of cycle slips respectively above or below the nominal frequency between the phase locked loop and the received signal.